1. Field of the Invention
The present invention relates to communication, and more particularly, a method and apparatus for modulating data.
2. Description of Related Art
Digital communication relies on numerous different, albeit related, forms of digital modulation such as phase shift keying (PSK), bi-phase shift keying (BPSK), quadrature phase shift keying (QPSK or 4-PSK), and quadrature amplitude modulation (QAM).
BPSK will be described with reference to FIG. 1. As shown, the magnitude of a reference carrier is constant, and to transmit either a 0 or a 1, the phase thereof is “keyed” or switched between 0° and 180°. A receiver then decides whether a 0 or a 1 was transmitted based on the phase of the received carrier, and generates the original data stream. With this simple scheme, one bit of information is transmitted with each state or symbol, so that the carrier phase is keyed at the data rate. FIG. 1 also illustrates the constellation for BPSK. As shown, the BPSK constellation diagram includes two points in the I-Q plane where I stands for in-phase (i.e., phase reference) and Q stands for quadrature (i.e., 90° out-of-phase). The two points in the BPSK constellation diagram represent the position of the signal at the “timing instance”. The timing instance is when the receiver interprets the signal. The signal can only be at one position at a time, but the constellation can be thought of as having persistence so that all of the proper states appear. Constellation diagrams such as in FIG. 1 typically do not show the transition between states and it should be noted that this transition does take a finite time. But for clarity, the transitions are not shown otherwise traces connecting the two states would clutter the diagram.
FIG. 2 illustrates the constellation diagram for QPSK. As shown, four different states exist in the QPSK diagram at phase values of 45°, 135°, 225°, and 315°. As further shown, each state corresponds to a symbol representing two bits. Because the data is taken two bits at a time to form a symbol, the symbol rate is half the bit rate. As a result, QPSK requires half the band width of BPSK for the same bit rate.
FIG. 3 illustrates that constellation for 16-QAM (quadrature amplitude modulation). In addition to modulating the phase, the amplitude of the signal is also modulated to create four distinct constellation points within each quadrant of the I-Q plane. As shown, in 16-QAM, one symbol represent four bits of data.